This invention relates to a photoelectric barrier, particularly useful as an accident-preventing device in machine tools.
Known are light barriers acting as safety devices in presses, wherein a light beam is moved periodically and in rapid succession through an area to be protected. Through suitable optical means, the light beam is redirected to a photoelectric transducer. If an obstacle enters the protected area, the output signal from the photoelectric transducer varies such that the press is stopped through a suitable electronic processor.
A known type of photoelectric barrier comprises a polygonal rotating mirror whereon a light beam is caused to impinge. The polygonal mirror is located at the focus of a concave mirror, such that the light beam, which is periodically diverted by the rotation of the polygonal mirror, is moved parallel to itself and scans periodically the area to be protected, which is included between said concave mirror, on one side, and a reflector reflecting the light beam to itself in every position, on the other side. Between the light source and the polygonal mirror, there is provided an inclined semitransparent mirror which diverts the reflected light beam to an electric receiver.
With that device, the area to be protected is scanned periodically at a high rate. When the area is clear, the light beam can travel back and forth undisturbed, and the receiver is at a state of substantial equilibrium. However, as an obstacle, e.g. the hand of an operator, enters the field of the protected area, a series of dark pulses appear on the receiver. Such pulses, through suitable electronics, cause a relay to open which stops the press.
It is a safety requisite that the press cannot be activated, or is shut off, while the photoelectric device is rendered inoperative by a failure whatever. For this reason, in conventional photoelectric barriers there is provided a monitoring device, effective to monitor the photoelectric barrier and its correct operation upon completion of each press stroke. To this end, during the safe stroke of the press pusher movement, i.e. during the upstroke thereof, the photoelectric barrier is subjected to full checking. Only if the checks are satisfactory will the press be permitted to effect the subsequent stroke.
However, this monitoring approach fails to eliminate the chance that after the checking has been carried out, i.e. during the downstroke of the press pusher, part of the safety arrangement is ineffective. If at this moment, one enters the protected area an accident may occur in spite of the monitored operation provided.
Also known is a photoelectric barrier (German Pat. No. 1221927), wherein the hazard just described is avoided in that the operation of the safety device is not checked after each stroke of the press, but rather after each scan of the protected area by the light beam. This is accomplished through a mirror-carrying polygonal rotor, wherein the mirrors are arranged alternately with blank regions, thereby after each protection scan (mirror region) there occurs a self-checking interval (blank region). The dark pulses can only appear by contrast to a preceding bright pulse, and if the height of the protected area is covered by an obstacle such that the photoelectric receiver receives no light, the pulses disappear. However, that known device also obviates this problem.
According to the German Pat. No. 1221927, the protection period is separated from the checking period by a short bright pulse. Such bright pulses are generated by means of small mirrors located on the mirror-carrying rotor, and reflect the light directly onto the photoelectric transducer. Thus, the bright pulses will still appear even when the protected surface is completely obscured. During the hazardous downward stroke of the press cycle, a protection phase occurs which is followed by a checking phase, characterized by the bright pulses which are self-generated by the mirrors, and so forth in synchronization. The self-checking principle disclosed in the cited German Pat. No. 1221927 shows no signal of a dynamic type up to the final relays, thereby a complementary failure of the final relay stages cannot be detected. That known device, moreover, employs a glow bulb type of light source, and the photoelectric receiver cannot, therefore, be shielded from light sources such as the sunlight and lamps.